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Pay-per-view television channels prove that humans will shell out cash to see sex and celebrities; now a new study reveals that monkeys also will pay to see sexy photographs and images of high-status individuals within their own social groups.

The study is the first to show that monkeys appraise visual information for its social value and can then use this data to spontaneously discriminate between images of their fellow monkeys.

Because many of the findings also apply to humans, the researchers say their findings could lead to a better understanding of neurophysiological disorders, such as autism, which affect how individuals view themselves and others.

Twelve adult male rhesus macaques (Macaca mulatta) participated in the study. Researchers observed the macaques, housed in captivity, to determine their social order. Males and females were either high-status or low-status relative to their cage mates.

"In our colony, when we give treats - for example, dried fruit - to the monkeys, the higher-status monkey usually gets the lion's share, and the low-status monkey will generally give a submissive gesture [such as lowering its head] in this potentially stressful situation," says author Dr Robert Deaner, a postdoctoral research associate in the centre's neurobiology department.

"Higher-ranking animals usually, but not always, enjoy priority of access for food, coveted spatial locations, and mating opportunities," he adds.

Photographing monkeys' behinds

Next, the researchers took photographs of the macaques and loaded them into a computer program. Some females were photographed from behind, so that the image showed a close-up of their hindquarters. Other pictures were head shots of both males and females.

Four of the male monkeys then sat in front of computer screens. They were rewarded with juice whenever they shifted their gaze from one image to another. Some images resulted in more juice than other photos.

When given the choice between a photo of a low-status male with a high juice reward and a photo of a female's hindquarters, the male test subjects refused the extra juice so that they could gaze at the sexy female images. They also "paid" with juice to see photos of high-status males.

Conversely, the male monkeys required extra payment, meaning more juice, to view the faces of low-status males and females.

"We think that the monkeys value orienting towards all kinds of social information, but some types of information are worth more than others," says Deaner.

The study determined that when males look at high-status males they pay attention, but do not make sustained eye contact. Deaner says long looks could signal aggression and, under normal circumstances, could pose a potentially costly threat of violence.

Males did not look away as quickly when they admired the females' behinds.

"Viewing the hindquarters of a female, on the other hand, may provide a benefit, but no potential cost," Deaner says, and added that males likely check out the colour and size of the female's perineum, or her genitalia. A large, red perineum generally means she is ovulating.

In another phase of the test, Deaner and his colleagues placed the male monkeys in front of mirrors. While animal experts do not believe that rhesus macaques possess self-recognition, high-status monkeys spent 41% of the mirror time looking at themselves, while low-status monkeys only gazed at their reflections for 19% of the session time.

How is money involved?

Professor Colin Camerer at Caltech is an expert on neuroeconomics, an emerging field that uses detailed evidence about brain mechanisms, including cross-species comparisons, to improve our understanding of human economic behaviour.

Camerer says it is "no surprise" that male monkeys "really like looking at female posteriors". But he is puzzled that males would pay with juice to see high-status males, but would not look at them for very long.

"It is like a star-struck fan who waits for hours to see a favourite movie star, say Brad Pitt, but then is so star-struck that she immediately averts her eyes downward shyly," Camerer says.

The human link to the monkey findings could extend to autism.

"One of the main problems in people with autism is that they don't find it very motivating to look at other individuals," says Deaner's colleague Assistant Professor Michael Platt. "And even when they do, they can't seem to assess information about that individual's importance, intentions or expressions."

Platt says he and his team in future hope to use the findings to not only learn more about monkeys, but also to model how social motivation is processed by both macaques and humans.